The present invention relates to electrolyte compositions for supercapacitors, also referred to interchangeably as ultracapacitors or electric double-layer capacitors (EDLC).
Supercapacitors store ionic charge electrostatically at the interface of high surface area electrodes, such as carbon electrodes, in a liquid electrolyte composition. Efforts to increase the energy density of supercapacitors have focused mainly on developing higher surface area electrodes and controlling electrode pore size. Energy density of supercapacitors can also be increased through faradaic mechanisms commonly known as pseudocapacitance, which arises from the introduction of redox active groups through functionalization of the carbon electrode surface or the incorporation of metal oxides.
Despite significant improvements in electrode materials design, most non-aqueous electrochemical capacitors use the same electrolyte compositions: either a mixture of tetraethylammonium tetrafluoroborate (TEABF4) in acetonitrile (MeCN) or TEABF4 in propylene carbonate (PC). These electrolyte compositions have a high specific conductivity that minimizes resistive losses and enables capacitors to operate at high power. However, these electrolyte compositions typically exhibit a practical voltage window around 2.7 volts, beyond which the capacitor lifetime is significantly reduced. Because the energy stored in a capacitor increases quadratically with voltage, extending the electrochemical window of the electrolyte composition could significantly improve the energy density of the capacitor.
Accordingly, there remains a continued need for an electrolyte composition that can extend the operating voltage window of a supercapacitor.